Wikidata is a sensitive topic. So before launching this feature, please document it and consider how you want to deploy it? I would advice to advertise users that they can opt-in for this feature in their Notifications preferences. Where deploy it is questioned on T142103.

I've also think about some possible questions or feedback that may be raised up by the users.

Some users would be very happy to have a preview of the Wikidata item title and description, for example by having them displayed in the dropdown. Possible?

Some users would prefer to have this notification as a line in the watchlist. Possible? If so, when?

Some users would like to have a notification when a page is connected to a different item: "The page X has been moved to a different idem"

Wikidata is a sensitive topic. So before launching this feature, please document it and consider how you want to deploy it? I would advice to advertise users that they can opt-in for this feature in their Notifications preferences. Where deploy it is questioned on T142103.

I've also think about some possible questions or feedback that may be raised up by the users.

Some users would be very happy to have a preview of the Wikidata item title and description, for example by having them displayed in the dropdown. Possible?

Some users would prefer to have this notification as a line in the watchlist. Possible? If so, when?

Some users would like to have a notification when a page is connected to a different item: "The page X has been moved to a different idem".

Which corresponds with the mission of this feature: make people aware of Wikidata ;)

So before launching this feature, please document it and consider how you want to deploy it? I would advice to advertise users that they can opt-in for this feature in their Notifications preferences. Where deploy it is questioned on T142103.

New users are opt-out, others opt-in (was this the decision, @Lydia_Pintscher?).

I've also think about some possible questions or feedback that may be raised up by the users.

Some users would be very happy to have a preview of the Wikidata item title and description, for example by having them displayed in the dropdown. Possible?

If Wikibase has services for this, it should be. But then the code will need some changes with compatibility handling.

Some users would prefer to have this notification as a line in the watchlist. Possible? If so, when?

Already possible in watchlist with Wikidata enabled.

Some users would like to have a notification when a page is connected to a different item: "The page X has been moved to a different idem"

I recall that the motivation for this kind of notification was to raise awareness about Wikidata but make as few noise as possible. Moreover, I think Wikibase doesn't recognize a "move" action.

Which corresponds with the mission of this feature: make people aware of Wikidata ;)

Yeah. So having mentions of Wikidata displayed everywhere is not why people are complaining, right? :)

So before launching this feature, please document it and consider how you want to deploy it? I would advice to advertise users that they can opt-in for this feature in their Notifications preferences. Where deploy it is questioned on T142103.

New users are opt-out, others opt-in (was this the decision, @Lydia_Pintscher?).

I recommend to opt-out for everyone:

people who don't want to have it will not have it

people who are not aware of Wikidata will not be disturbed by something they don't understand

I've also think about some possible questions or feedback that may be raised up by the users.

Some users would be very happy to have a preview of the Wikidata item title and description, for example by having them displayed in the dropdown. Possible?

If Wikibase has services for this, it should be. But then the code will need some changes with compatibility handling.

Some users would prefer to have this notification as a line in the watchlist. Possible? If so, when?

Already possible in watchlist with Wikidata enabled.

Some users would like to have a notification when a page is connected to a different item: "The page X has been moved to a different idem"

I recall that the motivation for this kind of notification was to raise awareness about Wikidata but make as few noise as possible. Moreover, I think Wikibase doesn't recognize a "move" action.

Thanks @matej_suchanek for your answers.
I also recommand to have the feature in opt-in (disabled by default, to be activated manually in the preferences), for all the users.

I completely understand the idea of having people more aware of Wikidata, what their impact is when they create an article, etc. But I also know that the newcomers have already a lot of information to deal with for understanding the global structure of the Wikimedia projects. I would then let then discover the projects step by step without pushing too much information.

Honestly, I also think that on Wikipedias where Wikidata is a sensitive topic, the announcement of this feature will be way easier if it's disabled by default.

What we can do instead is improving documentation, especially on Wikipedias, to include more information about Wikidata :)

I stand by my decision to have this be opt-out for new users. The whole goal of this is to help new people understand that they should connect their articles to Wikidata items and that they understand that Wikidata holds data about the topic they were writing about. If we do that opt-in we might as well not have this feature at all.

If you would use opt-out for existing users, I know that would gain a lot of negative attention for Wikidata on the Dutch Wikipedia (Wikidata is still a very sensitive subject there). For new users I don't think opt-out is such a big problem. They don't know what Wikidata is and don't have the feeling that they get something forced upon them by the developers once again. Exisiting users on the Dutch Wikipedia tend to get that feeling when something new comes, they didn't ask for and gets implemented by default with or without opt-out possibility. Trying to get awareness for Wikidata and to get Wikidata accepted isn't by forcing Wikidata-things on existing users.
This feature should be announced at least in the Tech News as that gets read a bit more than the Wikidata newsletter on the Dutch Wikipedia.

If you would use opt-out for existing users, I know that would gain a lot of negative attention for Wikidata on the Dutch Wikipedia (Wikidata is still a very sensitive subject there). For new users I don't think opt-out is such a big problem. They don't know what Wikidata is and don't have the feeling that they get something forced upon them by the developers once again. Exisiting users on the Dutch Wikipedia tend to get that feeling when something new comes, they didn't ask for and gets implemented by default with or without opt-out possibility. Trying to get awareness for Wikidata and to get Wikidata accepted isn't by forcing Wikidata-things on existing users.
This feature should be announced at least in the Tech News as that gets read a bit more than the Wikidata newsletter on the Dutch Wikipedia.

You will have the same issue on English and French Wikipedias as well. This is why I suggest to put it in the preferences, but not force opt-in. People can then enable it if they want.